The monitoring of gases and liquids has become increasingly important in diverse areas such as the processing industry, science, the medical sector, protection of the environment, the oil and gas industry and in general all places where safety needs to be guarded. Environmental effects that are measured concern the measurement of physical parameters and/or the detection of specific components in a gas or liquid. For many purposes it is desired that a sensor meets one or more of the following requirements: small, remotely operable, mobile, high sensitivity, low detection limit, high robustness, small response time, high selectivity, large dynamic range, high accuracy.
Examples of sensors that perform well with respect to one or more of these requirements are optical sensors and microelectromechanical system (MEMS) cantilever chemical sensors.
Advantages of optical sensors include their easy operation on large distances, their small size, their flexibility and/or the possibility to make a sensor system consisting of an array of discrete sensors that all may be read separately from a single optical fibre.
One specific advantage of optical sensors over electronic measuring systems is that optical sensors are usually not adversely affected by the electromagnetic radiation that is generally produced in for example power cable systems, induction furnaces or equipment for nuclear magnetic resonance measurements, such as MRI or NMR equipment.
Optical sensors usually comprise a waveguide to transport the data of the measurement in the form of a specific spectrum of light. One principle on which an optical sensor system may be based is an axial strain of the waveguide, as a result of an environmental effect that is to be detected, for example by using a coating on the waveguide that deforms under the influence of the environmental effect. When a waveguide grating, guiding a specific spectrum of light, stretches or shrinks under such axial strain, the spectral pattern of transmitted light and/or the spectral pattern of reflected light (i.e. the spectral response) changes. Such changes in the spectral response provide—when measured—quantitative information on the environmental effect.
Typical sensor systems that are based on waveguide grating are, e.g., described in detail in U.S. Pat. No. 5,380,995, U.S. Pat. No. 5,402,231, U.S. Pat. No. 5,592,965, U.S. Pat. No. 5,841,131, U.S. Pat. No. 6,144,026, US 2005/0105841, U.S. Pat. No. 7,038,190, US 2003/156287.
US application 2005/0105841 relates to the use of a polyethyleneimine (PEI) monolayer coating on a Long Period Grating waveguide. The coating swells under the uptake of water, which makes a sensor comprising such coating suitable for measuring relative humidity (RH), based on changes of the refractive index of the coating. However, changes in refractive index are not selective for the detection of water, which makes the sensor sensitive to environmental pollutions. The preparation of the sensor is cumbersome due to the slow deposition of the monolayer. Also, the response time is relatively long, especially at a high humidity, and it appears that very high humidities cannot be measured, which results in a small dynamic range of the sensor. The refractive index of the coating should be tuned to the specific waveguide grating and therefore cannot be generally used on other waveguides. Thus, the technology of refractive index sensors is mainly limited to Long Period Grating waveguides, and such waveguides cannot be used in long multiple sensor waveguides.
A thesis by J. L. Elster (“Long Period Grating-based pH sensors for corrosion monitoring, Blacksburg, Va., 1999”) relates to a poly-acrylic acid coating on a Long Period Grating waveguide, which was applied to constitute a pH sensor. Such pH sensors are based on a change in refractive index of the coating around the cladding due to changes in the H+-concentration. Such sensors have disadvantages similar to those of the relative humidity sensor described US application 2005/0105841.
U.S. Pat. No. 7,038,190 relates to an optical humidity sensor making use of medical grade polyurethane foam or polyimide to sense humidity. Amongst others, the application describes to provide a fibre with an epoxy acrylate, that has a similar thermal response to polyimide but is relatively insensitive to humidity. Thus, in combination with polyimide it can be used as a fibre grating filter, to correct changes in signal of the grating coated with polyimide due to changes in temperature. Due to the thickness of the polymer layers, the response time is long (hours). No information is given concerning the preparation and the specific properties of the sensors.
Advantages of MEMS cantilever chemical sensors include their small size and/or their accuracy. A principle on which a MEMS cantilever chemical sensor may be based is a change in the properties of the cantilever element as a result of an environmental effect that is to be detected, for example by using a coating on the waveguide that changes the mass, stress, electrical or thermal properties of the cantilever element under the influence of the environmental effect. Such changes provide—when measured—quantitative information on the environmental effect.
Typical sensor systems that are based on a MEMS cantilever are, e.g., described in detail in Journal of Colloid and Interface Science 316 (2), pp. 687-693 and Materials Today 5 (1), pp. 22-29. The responsive layers on the cantilevers show a change in mechanical properties due to absorption of an analyte. However, a large amount of analyte molecules have to be absorbed before a change is noticed.